The present invention relates to the use of unsaturated peroxides in cross-linking processes, such as the cross-linking of silicone rubbers and the curing of unsaturated polyester resins.
The use of peroxides for the radical cross-linking of silicone rubbers is well-known, particularly for use in hot air vulcanization systems. Typically, products like bis(2,4-dichlorobenzoyl) peroxide, bis (p-chlorobenzoyl) peroxide, 2,5-di-tert-butylperoxy hexane, tert-butyl cumyl peroxide, dicumyl peroxide, and di-tert-butyl peroxide were used. All of these products are known to result in blooming of low-molecular weight products or smell, the latter probably given off by decomposition products of the peroxide, which is highly undesired. Although a temperature treatment of the cross-linked silicone rubber may (partially) prevent blooming from occurring, because the low-molecular weight products are stripped away, such a treatment is undesired, since it is energy consuming and may lead to discolouration of the cured material.
In EP-A-0 282 130 it is disclosed to use bis(2-fluorobenzoyl) peroxide for the cross-linking of a silicone rubber in order to attain a good curing rate and to prevent decomposition products from blooming. Similarly, EP-A-0 235 537 proposes to use specific bis(4-alkylbenzoyl) peroxides for the cross-linking of silicone rubber. These bis(4-alkylbenzoyl) peroxides are said to give stable dispersions when formulated with silicone oils and not to result in discolouration of the cross-linked silicone rubber even when the dispersions have been stored for a prolonged period. However, the nature of these peroxidesxe2x80x94they all have a dibenzoyl peroxide skeletonxe2x80x94inherently leads to the formation of aromatic decomposition products. Such products are undesired, since they are known to result in a bad smell of the cured product. DE-A-1 932 475 teaches to use tert-butylperoxy crotonate for the vulcanization of carbon black-filled organosiloxanes. However, the use of tert-butylperoxy crotonate was found to result in a silicone rubber with unacceptable properties, especially when cross-linking occurred at lower temperatures (below 150xc2x0 C.).
For the radical curing/cross-linking of unsaturated polyester resins many peroxides are used, the selection typically being dependent on, inter alia, the composition of the formulation, processing temperatures, the desired reaction times, and residual monomer levels. For processes which are conducted at higher temperatures ( greater than 70xc2x0 C.) very often di(4-tert-butylcyclohexylperoxy) peroxydicarbonate is used to get a quick gelation of the formulation. However, the product is not readily soluble in the unsaturated polyester resin and its use results in high residual styrene levels in the cured resin. Another frequently used peroxide is tert-butylperoxy benzoate. This peroxide, when used alone, leads to acceptably low residual styrene levels at curing temperatures above 110xc2x0 C., but its use often results in too long gel times and it also contains the undesired benzoyl moiety, leading to benzene formation upon decomposition. A combination of di(4-tert-butylcyclohexyl) peroxydicarbonate and tert-butyl peroxybenzoate, which is known to be used, renders a system with acceptable gelation times and residual monomer levels. However, such a combination also suffers from the solubility and decomposition product problem. Many other combinations of peroxides have been proposed for use as cross-linking initiating systems. However, the use of combinations of peroxides is undesired since storage of two or more types of peroxides, typically with different storage temperature requirements, is troublesome and because handling and mixing is more time-consuming and metering a source of errors.
Therefore, there has been a continued search for alternatives and improved products for use in the field of cross-linking processes. More particularly, the industry is in need of new types of initiators that will give an effective cure of silicone rubbers and unsaturated polyester resins, comparable with the curing of conventional peroxides, but do not suffer from blooming, bad smell, solubility problems, and the like. Preferably, the alternatives can be used at conventional lower temperatures of up to 160xc2x0 C. Also they are preferably used as the sole initiator, without that it is needed to combine them with other initiators.
Surprisingly, we have found that specific peroxides according to the invention can be used to fulfill this need. The specific peroxides are characterized by the formula 
n=0 or 1,
R1, R2, and R3 are independently selected from the group consisting of hydrogen, C1-C20 alkyl, C2-C20 alkenyl, C2-C20 alkynyl, C3-C20 cycloalkyl, C3-C20 cycloalkenyl, C6-C20 aryl, C7-C20 aralkyl, and C7-C20 alkaryl, which groups may include linear or branched alkyl moieties and which optionally are substituted with one or more groups selected from hydroxy, alkoxy, linear or branched polyalkyloxy, aryloxy, halogen, ester, ketone, carboxy, nitrile, and amido. Any pair of R1-R2, R1-R3, and R2-R3 may be linked to form a cyclic structure. R1,R2, and R3 may also be linked to form a polycyclic structure, and R4 is selected from the group of radicals consisting of C1-C20 alkyl, C2-C20 alkenyl, C2-C20 alkynyl, C3-C20 cycloalkyl, C3-C20 cycloalkenyl, C6-C20 aryl, C7-C20 aralkyl, and C7-C20 alkaryl, which groups may include linear or branched alkyl moieties and which optionally are substituted with one or more groups selected from hydroxy, alkoxy, linear or branched polyalkyloxy, aryloxy, halogen, ester, ketone, carboxy, nitrile, and amido, and radicals of formulae II and III, 
m=0 or 1,
R5 is selected from C1-20 alkylene, C1-20 polyoxyalkylene, C2-20 alkenylene, C2-20 polyoxyalkenylene, C2-C20 alkynylene, C2-20 polyoxyalkynylene, C3-C20 cycloalkylene, C3-C20 cycloalkenylene, C6-C20 arylene, C7-20 aralkylene, and C7-20 aralkenylene groups, which groups may include linear or branched alkyl moieties and all of which may optionally contain one or more hetero atoms, and wherein R6, R7, R8, and R1xe2x80x2, R2xe2x80x2, R3xe2x80x2 are independently selected from the group consisting of compounds according to the definition of R1, R2, and R3, respectively.
Preferably, R1, R2, R3, R1xe2x80x2, R2xe2x80x2, R3xe2x80x2, R6, R7, and R8 are independently selected from the group comprising hydrogen, methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, pentyl, hexyl, heptyl, nonyl, and phenyl. The various R groups can be varied in such a way that a product with the desired properties is obtained, e.g. a certain melting point/range. Alternatively, mixtures of peroxides of formula I can be used that have the desired properties.
More preferably R1, R2, R3, R1xe2x80x2, R2xe2x80x2, R3xe2x80x2, R6, R7, and R8 are selected such that one or more of the following moieties is formed: 
H3Cxe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94 (any isomer), and Alkylxe2x80x94Oxe2x80x94COxe2x80x94CHxe2x95x90CHxe2x80x94 (any isomer).
R5 preferably is xe2x80x94Cxe2x95x90Cxe2x80x94Cxe2x95x90Cxe2x80x94 (the residue of muconic acid) when n and m are 0, and C2-12 alkylene or C2-12 polyoxyalkylene, when both m and n are 1.
More preferred peroxides for use in the curing process of the present invention are:
di-2-butenoyl peroxide (all isomers)
di-2-methyl-2-butenoyl peroxide (all isomers)
di-3-methyl-2-butenoyl peroxide (all isomers)
(Z,Zxe2x80x2)-bis(4-butoxy-4-oxo-2-butenoyl) peroxide
di-2-butenoyl muconoyl peroxide of formula 
2-butenoyl peroxy alkylcarbonates, of formula 
and
di (2-butenoylperoxy) alkylenedicarbonates, such as 
The peroxides of the invention can be produced in conventional ways by reacting the appropriate acid chloride(s), chloroformate, (mixed) anhydride, or the like, with sodium peroxide. How to use mixed anhydrides in the process to make the unsaturated peroxides is explained in more detail in non-prepublished PCT patent application PCT/EP00/09927.
Depending on the peroxide in question, it is possible that it can be used in the pure form. However, because of safety and/or handling considerations, it can be preferred to formulate the peroxide in an acceptable manner. Accordingly, when speaking of peroxide in this document, such term is considered to encompass the pure peroxide as well as any formulation thereof. Examples of suitable formulations are solutions, pastes, and/or dispersions of the peroxide in a conventional phlegmatizers, or powders, granulates or masterbatches as obtainable by combining any of the pure peroxides, solutions, pastes, and/or dispersions with a conventional carrier material. If a paste of a solid peroxide is used, it is preferred that the peroxide particles have an average particle size less than 50 xcexcm, preferably less than 30 xcexcm, for the reasons as presented in EP-A-0 939 103. Furthermore, it is preferred that such pastes are dispersions of the peroxide in a silicone oil or low molecular weight polysiloxane. Powdery formulations preferably contain conventional inert fillers, such as silica, calcium carbonate, kaolin, and the like. Masterbatch formulations comprise the peroxide and an inert polymer or rubber that is compatible with silicone rubber. Preferably, peroxide masterbatches for use in silicone rubber curing comprise a silicone rubber. If so desired, granulates and masterbatches may also comprise fillers and or liquid phlegmatizers.
The peroxides of the invention are preferably used in curing processes that are conducted at a temperature from 50 to 250xc2x0 C. More preferably, the curing process is conducted at a temperature from 70 to 200xc2x0 C., and most preferably at a temperature from 80 to 150xc2x0 C. Where the heat transfer coefficient with the processing equipment is poor, such as in hot air curing processes, or in unsaturated polyester mouldings with a thickness of over 5 mm, the peak exotherm that is observed on the inside or the outside of the product being cured may be higher. The curing temperature, therefore, is the average temperature of the whole of the product being cured over the total curing time.
The term xe2x80x9csilicone rubberxe2x80x9d as used throughout this document is meant to designate any compositions comprising an organopolysiloxane. The polysiloxanes are typically based on dimethyl polysiloxane, dimethylphenyl polysiloxane and/or copolymers of these polysiloxanes and typically have a molecular weight ranging from 5,000 up to 750,000 Dalton. They may comprise monomeric units of the structure 
and/or 
They may be blocked, random and/or terpolymers. For a further description reference is made to W. Hofmann, Rubber Technology Handbook, ISBN 3-446-14895-7 Hanser Publishers, pp 129-136. The silicone rubber may contain conventional additives. Typical additives that are used in silicone rubbers include reinforcing fillers, such as silica; pigments; stabilisers; plasticizers, such as silicone oils or uncured silicone rubber; and products like methyl hydrogen polysiloxane. If so desired, one or more of these additives may be used in making a peroxide formulation according to the invention.
The cured silicone rubbers can be used in a variety of applications, including electrical, electronic, automotive, textile, and pharmaceutical/medical components.
The more preferred peroxides for use in the process to cure silicone rubbers are (substituted) 2-butenoyl alkanoyl peroxides, (substituted) 2-butenoyl alkenoyl peroxides, (substituted) 2-butenoyl peroxy alkylcarbonates, and di-(substituted)-(2-butenoylperoxy) alkylene(oxy) dicarbonates. Most preferred peroxides in this process are di(2-butenoyl) peroxide and di(2-butenoylperoxy) hexylenedicarbonate. For curing silicone rubbers it can be advantageous to combine one or more of the peroxides of the invention with one or more conventional peroxides. If a combination of peroxides (pure or formulated) is used, they may be added separately to the silicone rubber, or be used in a premixed form. If so desired, trace amounts of platinum can be used together with the peroxide, as described in, for instance, EP-B-0 801 111.
The term xe2x80x9cunsaturated polyester resinxe2x80x9d is meant to encompass any conventional curable unsaturated polyester compound, as well-known in the art, including those systems wherein unsaturated vinyl esters are used. Typically, the polyester or unsaturated vinyl ester is combined with one or more polymerizable monomers together with further optional additives to make the unsaturated polyester resin.
Conventional vinyl esters, as well-known in the art, typically are compounds comprising a xe2x80x94C(O)xe2x80x94OCH2CH(OH)CH2Oxe2x80x94 moiety and terminal, polymerizable vinylidene groups. Typical vinyl ester resins include the reaction products of dicarboxylic acid half esters of hydroxy (meth)acrylates and polyepoxide resins, the reaction products of glycidyl(meth)acrylates and salts of polyols, and the reaction products of (meth)acrylic acid and epoxy novolac resins. The polyepoxides typically are the glycidyl polyethers of polyols, including polyhydric phenols.
Conventional unsaturated polyesters, as well-known in the art, typically are the condensation products of unsaturated hydroxyacids, the reaction products of carboxylic acids with two or more acid groups, or anhydrides, and polyols having two or more hydroxyl groups per molecule, or the reaction products of a mixture of hydroxyacids, carboxylic acids, and polyols. By a proper selection of raw materials, the desired unsaturation of the resulting resin is obtained.
Suitable monomers are of the conventional type and include, but are not limited to, styrene, vinyl toluene, divinyl benzene, (meth)acrylic acid esters, vinyl acetate, diallylmaleate, and dimethallyl fumarate.
The unsaturated polyester resin is typically cured to form a cross-linked product (a thermoset) by means of a radical polymerization process. Depending on the temperature at which the resin is to be cured, various peroxides are known to be used, optionally together with accelerators and optionally in a redox system. In the unsaturated polyester resin curing process of the invention, at least one peroxide is used satisfying the definition given in claim 1. The most preferred peroxides for use in the curing process of unsaturated polyester resins are di-2-butenoyl peroxide (all isomers) and di-3-methyl-2-butenoyl peroxide (all isomers).
Experimental
Di(2-butenoyl) peroxide was prepared using the following procedure: 0.20 mol trans-crotonyl chloride was dropped (in 50 min.) into a solution of 0.115 mol sodium peroxide in water. The temperature was kept at 0-10xc2x0 C. and the pH was maintained at a value of about 10 by addition of sodium hydroxide (25% in water). After addition of the crotonyl chloride, the mixture was post-reacted for 40 minutes at 10xc2x0 C. Thereafter, the product was extracted with 50 ml diethylether. The organic layer was washed three times with a 25% solution of sodium chloride in water, dried with magnesium sulfate, filtered, and concentrated in vacuum to yield 5 grams of peroxide of 75% purity.
Perkadox(copyright) PD-50Sps=Di(2,4-dichlorobenzoyl) peroxide, ex Akzo Nobel
Perkadox(copyright) PM-50Sps=Di(4-methylbenzoyl) peroxide, ex Akzo Nobel
Siloprene(copyright) HV3/611U (silicone rubber), ex Bayer
Siloprene(copyright) HV3/711U (silicone rubber), ex Bayer
In the examples where silicone rubber is cured, the peroxides and silicone rubber were first mixed using a two-roll mill, Dr. Collin, at room temperature (about 25xc2x0 C.) to form mixed compositions. The mixing on the two-roll mill occurred without the incorporation/formation of voids in the compound.
The curing characteristics of the mixed compositions were determined using a Monsanto Rheometer(copyright) MDR 2000E. The temperature was varied, depending on the half-life of peroxide(s) used.
Values for ts2 (scorch time in minutes), t90 (cure time in minutes), and the delta TQ (increase of torque during curing in Nm) were recorded.
The mixed compositions were pressed into a sheet-type form using a stainless steel 316 mould with dimensions 105xc3x97180xc3x972 mm using a Fonteyne press. The sheets were cured either by compression moulding, using the same press and mould, at 110xc2x0 C. for 15 minutes, or by hot air curing the sheet in a Heraeus circulation air oven at 200xc2x0 C. for 6 hours.
The tensile properties of the cured sheets were determined in accordance with ISO 37-""77, using a Zwick tensile tester.
The compression set of a cured rubber sheet was determined in accordance with ISO 815-""91 at 175xc2x0 C. for 24 hours.
Blooming was determined visually.
The curing of polyester resins was performed in accordance with method F 77.1 as published by the Society of Plastic Institute. Time-temperature curves were measured at 100xc2x0 C. on compounds containing 100 parts of a standard polyester resin (Ludopal(copyright) P6, ex BASF), 150 parts of conventional sand with a particle size of 02-0.63 mm, ex Sibelko (as a filler), and the required peroxides. The peroxide amount was adjusted such that it equated, on active oxygen (Act. O) basis, to 1 part by weight of tert-butyl peroxybenzoate (Trigonox(copyright) C ex Akzo Nobel). In the test 25 g of compound are poured into a test tube and a thermocouple is mounted through the enclosure cork at the centre of the tube. The glass tube is then placed in the oil bath maintained at the test temperature and the time-temperature curve is measured. From the curve the following parameters were calculated:
Gel time (GT)=time in minutes elapsed between 16.7xc2x0 C. below and 5.6xc2x0 C. above the bath temperature. 
Time to peak exotherm (TTP)=time elapsed between the start of the experiment and the moment that the peak temperature is reached. 
Peak exotherm (PE)=the maximum temperature which is reached. 
The residual styrene level was determined by conventional GC techniques after dissolving the sample in dichloromethane.